Extensive System

The extensive system is characterized as having grassland or rangeland as the main source of feed, and grazing is carried out without control and independently of forage availability and stocking rate, mineral salt supplementation and limited or null health management (^{Cuéllar et al. 2012}). To find the elements necessary to meet their nutritional needs, animals walk large distances. The nutritional level of sheep based on tropical grass diets has been low, as these diets only cover the requirements for maintenance with a reduced surplus for productive functions. An additional problem in these systems is the presence of gastrointestinal parasites, such as Haemonchus contortus, Trichostrongylus colubriformis, Cooperia curticei and Moniezia expansa (^{López et al. 2013}). ^{Nuncio-Ochoa et al. (2001)} named this type of system as traditional extensive, characterized by a diversified production, combining sheep production, crops, cattle, poultry, and backyard pig production, scant economic reinvestment, high use of local resources and low use of external resources. In these systems, an average of 53 g of live weight gains (LWG) can be achieved, and the sheep are commercialized when they reach a live weight (LW) of approximately 25 to 30 kg at the age of one year or more (^{Macedo and Castellanos 2004}). The objective of this system is the production of animals for slaughter; hence, mutton is the main product (^{Pérez et al. 2011}).

Semi-intensive System

In this production system, ewes are allowed to graze and can be supplied with a strategic feed supplementation (^{Galaviz-Rodríguez et al. 2011}). Lambs are fed with maternal milk and occasionally supplied with concentrate feed by creep feeding systems. Under these conditions, LW of approximately 18 kg at weaning can be achieved at an age of 84 d with Pelibuey genotypes (^{Macedo and Castellanos 2004}). There are farms where, after weaning, the lambs are fattened in confinement conditions or with a mixture of grazing and concentrate feed supplied after grazing in the stable (^{Góngora-Pérez et al. 2010}). Unlike extensive systems, these farms are characterized by specialized sheep production, with economic reinvestment and moderate use of local and external inputs. The objective of these systems is the production of animals for slaughter or breeding (^{Pérez et al. 2011}).

Intensive System

Under these systems, lamb production for slaughter or breeding is more intensive, with high use of inputs and technology (^{Góngora-Pérez et al. 2010}). To increase and improve mutton production in as short a time as possible, the animals remain in confinement all the time (^{Partida et al. 2009}). Therefore, the animals must be supplied with the amount and quality of feed necessary to meet their nutritional requirements (^{Canton et al. 2009a}). This system, although efficient, has a high level of investment in buildings and capital (^{González-Garduño et al. 2013}). One limiting factor is the lack of animals suited for the goal pursuit (^{Cuéllar et al. 2012}). Although some farms have the goal to produce breeding stock, the main objective is the production of animals for slaughter (^{Góngora-Pérez et al. 2010}, ^{Pérez et al. 2011}).

Feed resources

The lambs aimed for slaughter are fattened in intensive systems, based on grain feeding, as this strategy meets the nutritional requirements to achieve both better LGW (^{Fimbres et al. 2002a}) and better carcass characteristics (^{Fimbres et al. 2002b}) and to reduce the fattening period (^{Partida et al. 2009}). Nonetheless, the increment in cereal grain prices is a threat to the sustainability of such production systems, (^{Rihawi et al. 2010a}, ^{Hartwell et al. 2010a}, ^{Hartwell et al. 2010b}). On this matter, it has been demonstrated that supplementation with concentrate or foliage of shrub species after grazing improves carcass yield and other carcass characteristics. Thus, production costs are decreased. This strategy has been rarely used in Mexico; however, several authors have reported the viability of this feeding system for lamb fattening (Table 1). For instance, ^{Archiméde et al. (2008)}, observed an increase in feed consumption and digestibility with supplementation of concentrate feed. ^{Rihawi et al. (2010)} noted that semiintensive systems reduce feeding costs and promote better growth of lambs compared with lambs fed with grains in an intensive system and those under grazing with supplementation.

Table 1. Initial weight (IW), final weight (FW), daily weight gain (DWG), carcass weight (CW) and carcass yield (CY) of lambs under different feeding systems. 

C, 0 = Cynodon nlemfuensis, R. C-H = Relation Cynodon nlemfuensis-Hibiscus rosa-sinensis, Gs = Gliricidia sepium, Hr = Hibiscus rosa-sinensis, Gd = Chick peas (discarded from human consumption) in intensive systems. ^a,b,c, means in the same column with different superscripts indicate significant differences.

Another feed alternative with great potential for growing lambs is foliage of various shrub species such as Hibiscus rosa-sinensis and Morus alba, which provide similar LWG to that of concentrate feeds. ^{Mata et al. (2006)} report that production performance of lambs fed with Hibiscus rosa-sinensis is similar to the performance of animals fed with concentrate. The use of this shrub (as approximately 60 % of the diet) improves the digestibility of the provided diet and also the productivity of the animals (^{Ruiz-Sesma et al. 2006}). ^{Obrador-Olán et al. (2007)} reported a better daily gain (131.6±4.3 g) when animals were supplemented with concentrate in addition to grazing; however, these authors propose a harvest of Morus alba and Hibiscus rosa-sinensis leaves between 60 and 90 days to obtain daily gains of 90.5±4.3 and 107.9±4.4 g, respectively. Chick peas (discarded from human consumption) were also tested as an alternative feed supplement for intensive systems for finishing lambs. The results indicate an improvement between 15-30 % in production; therefore, it can be seen as a possible alternative feeding source to reduce feeding costs (^{Rios et al. 2012}). ^{Duarte and Pelcastre (1998)} showed that cassava (Manihot esculenta) can replace corn as an energy source with similar results in daily gain (276 and 283 g d⁻¹ for corn and cassava, respectively).

There are many other available feed resources in the tropics of potential use in ruminant feeding, particularly in the development of food-feed systems that are not only beneficial for human and animals but also for the environment (^{Wanapat 2009}). The inclusion of energy sources such as molasses and citrus byproducts has been suggested to increase microbial protein synthesis and volatile fatty acid production, which results in improved animal performance (^{Ramírez et al. 2006}, ^{Salinas-Chavira et al. 2011}). Nonetheless, feeding ruminants with large amounts of tree foliage, in addition to not being economical, could result in a substitution effect (^{Ramírez et al. 2006}), which can also be observed when large amounts of grain are included in diets. This finding suggests that for the inclusion of high-quality foliage and concentrates, the feed must be supplied in suitable quantities. According to the above, the use of foliage can be a viable option for lamb fattening production (^{Ekiz et al. 2012}).

Animal genetic resources

Regarding breed use, Pelibuey (Pb), Black belly (Bb), Dorper (D) and Katahdin (Kt) have been the most used (^{Martínez-Partida et al. 2011}, ^{Pérez et al. 2011}, ^{Góngora-Pérez et al. 2010}). The Pb breed is used in several types of production systems (^{Herrera et al. 2008}) and, together with the Bb breed, has shown a great rusticity and adaptation, good reproductive rate, less breeding seasonality and less fat content in the carcass (^{Cantón et al. 2009}). Because of their precocity, fertility rate, and prolificacy, the Pb and Bb breeds should be advantageous for mutton production. However, their low growing and finishing rates do not permit these breeds to achieve a better productive potential, particularly when production takes place in extensive systems (Huerta 2008). Extensive systems only allow modest production, which conflicts with the current economic condition of an increasing demand for higher meat yield. The D and Kt breeds, either as pure or crossbreeds, represent a viable alternative as part of fattening programs in regions with humid hot climates (^{Hinojosa-Cuéllar et al. 2009}) and in the arid zones of Northeast Mexico (^{Macías-Cruz et al. 2010}). In sub-tropical regions, the productive response of these breeds and their crosses with Pb do not appear to have important differences when compared with the pure Pb breed (Table 2). However, there is a need to move forward in the evaluation of this type of crossbreeding for an assessment of the effect of the F1 genotypes on productivity and carcass characteristics in these regions.

Table 2. Daily weight gain (DWG), final weight (FW), carcass weight (CW) and carcass yield (CY) of Pelibuey (Pb) lambs and their crossbreeds with specialized breeds for mutton production. 

Pb = Pelibuey, D = Dorper, Kt = Katahdín, Db = Black head Dorper, Dw = Dorper White, D = Dorper (type not specified). ^a,b,c means in the same column with different superscripts indicate significant differences.

The differences between breeds and sex in conformation and fatness are evident in different studies (^{Partida et al. 2009}, ^{Vázquez et al. 2011}). There are breeds with early maturity (D, Kt) and late maturity (Pb) that define the pattern of adipose tissue deposition (visceral fat, subcutaneous, intramuscular or marbling). For example, the Pb breed, due to its rusticity, tends to accumulate more visceral fat compared with the Kt breed, which accumulates more fat in the carcass (^{Canton et al. 2009}). There is disagreement regarding productive performance and carcass composition attributes of hair sheep in the sub-humid tropic, which makes it diffcult to decide which breed or crossbreed is the right option for fattening systems in the region.

In other countries, the capacity of different genotypes to achieve the commercial goal in accordance with the seasons that limit or might limit growth is taken into account (^{Álvarez et al. 2013}). This suggests that in an evaluation of available genetic resources it is important to determine the breed or crossbreed that can accomplish a specific production and market goal, above other subjective issues that farmers could prefer. In the sub-humid tropic, farmers are focused on terminal crossbred animals such as Pb x Bb x Dand Pb x Kt x D (^{Góngora-Pérez et al. 2010}). The criteria for the implementation of such crossbreeding schemes are unknown, and there is no evidence in the literature indicating the productive performance of such crossbreds. The use of Kt x Pb genotypes could offer important productive yield (^{Canton et al. 2009}, ^{Macías-Cruz et al. 2010}) and economic profit in tropical regions and in other regions with similar climatic characteristics, as both breeds possess the characteristics designed for mutton production: middle size, strength and muscularity, early puberty, and long productive life. Additionally, these genotypes are prolific, with excellent maternal ability, good milk production and possible resistance to parasites (^{Esteban-Andrés et al. 2013}).

A new emerging fattening system

In recent years, lamb production under intensive fattening systems in (SRC) has emerged. Some authors mention that these systems allow adequate comfort to the animals, which in turn results in better growth, body development and health (^{Lupton et al. 2007}, ^{Hernández-Cortazar et al. 2014}). Such asseveration has caused the development of projects aimed to implement this type of infrastructure for lamb fattening. A tangible example of this model is found in the state of Tabasco, Mexico. However, there are no scientific reports on lamb productive performance and the profitability of fattening systems using SRC compared to conventional pens on the ground. It is possible that fattening systems in SRC can accomplish the features stated (^{Lupton et al. 2007}) but suggesting such a type of technology without a scientific basis can be a mistake. In addition, the cost of that infrastructure plus the cost of feeding, could cause the recurrence of farmers using intensive production systems in pens at ground level, grassing systems or even abandoning the activity.

Profitability of lamb fattening systems

Profitability can be defined as the net income on the total costs of production (fixed and variable), as the net income on the variable costs for a farm or by input unit such as one hectare or one cow (^{Nicholson 1998}). Thus, sheep production systems and the management decisions regarding them play an important role in profitability and flock dynamics. Livestock production involves complex systems that are not always easy to understand or achieve, including production processes, financial processes or a combination of both (^{Turner et al. 2013}). For example, when different resources are used (land, labor or feeds) in production, a greater productivity by resource unit (such as land or feeds) does not necessarily imply greater profitability (^{Nicholson 1998}). Understanding how associated changes in production decisions can affect a system's profitability in the long term is not an easy task. An analysis of production systems could be an effective tool to alleviate those risks (^{Turner et al. 2013}).

It has been argued that technologies and practices that provide the basis for the intensification of production systems could be successfully transferred if they are managed with ability and with a minimum of additional investments (^{Nicholson 1998}). In Mexico, the increase in demand for mutton has led to a change in the traditional farm structure. At present, sheep production systems that are characterized by a high level of investment in infrastructure and equipment and by the use of high amounts of grains have emerged (^{Góngora-Pérez et al. 2010}, ^{Pérez et al. 2011}). The use of SRC and commercial concentrated feeds are two concrete examples of the adoption of technologies or practices considered to be beneficial by farmers. However, these technologies could have a negative effect on the profitability of the production system as production costs rise. ^{Macedo and Castellanos (2004)} indicated that the cost of a weaned lamb and its feeding until finished represents the highest cost of production, 50 and 43 %, respectively. Labor, health practices and financial costs represent the last 7 %. These authors calculated the total cost for production of a single lamb at $37.45 USD and the total value of production at $49.41 USD, resulting in 1.32 for the cost/benefit ratio. ^{González-Garduño et al. (2013)} also observed that the highest production cost was the feeding (89.9 %). They concluded that lamb production using high levels of concentrated feeds for intensive systems is unprofitable, and they proposed grazing plus supplementation as a strategy to increase profitability in lamb fattening systems.

Several studies around the world support the idea that the feeding cost represents the greatest cost in livestock production systems (^{Hartwell et al. 2010a}, ^{Hartwell et al. 2010b}, ^{Theodoridis et al. 2012}, ^{Turner et al. 2013}); hence, the feeding practice that is utilized is one of the major factors affecting profitability. With regard to infrastructure, specifically SRC, there is no evidence of its profitability despite the great number of farmers that use it as a way to increase production, although its efficiency could imply the use of high quality animals.

According to ^{Theodoridis et al. (2012)}, the technical efficiency level is used as a criterion for systems classification. Accordingly, the main productive and economic indicators of farms are analyzed to elucidate their contribution to the improvement of technical efficiency. In this sense, the intensive fattening systems in SRC can be considered technical efficiencies. For example, the population density in such cages can be raised to two animals per m² (0.5 m² animal)⁻¹, and this change could increase profitability by reducing the cost per animal reared (^{Toro-Mujica et al. 2012}).

Additionally, it is unknown whether high density is in fact used with this type of system or its effect on animal productivity. Therefore, despite the belief that fattening systems in SRC can yield large production volumes, this is not a warranty of return on the investment in infrastructure for the short or middle term. The efficient use of existing farm resources is essential for an improvement in profitability and hence in competitiveness. In this context, measurement of efficiency can be a tool of practical decision-making for the adoption of strategies that might guide the producers to increase productivity. The productive and economic analysis could allow the detection of factors that can influence both the productivity and the profitability of the farms (^{Theodoridis et al. 2012}). In the same way, the analysis of different technological options that could be potentially useful for farmers can be an efficient tool to mitigate the risk of production and financial perspectives (^{Turner et al. 2013}).